The present invention relates to a projection exposure apparatus and, more particularly, to a projection exposure apparatus and a method for the exposure of a pattern of a mask onto a photosensitive substrate through an optical projection system, said pattern of the mask being so adapted as to be employed for manufacturing semiconductor elements, liquid crystal display elements and the like in a lithographic process.
In a lithographic process for manufacturing micro devices such as, for example, semiconductor elements, liquid crystal display elements, charge couple devices (CCD's ), thin film magnetic heads and the like, there has been employed a projection exposure apparatus for projecting an image of a photomask or reticle (hereinafter referred to generally as "reticle") with a transcribing pattern formed thereon onto a substrate such as, for example, a wafer or a glass plate (hereinafter referred to as "wafer") with a photoresist material coated thereon through an optical projection system.
The projection exposure apparatus of this kind requires a reticle to be aligned with a wafer at a high degree of precision prior to exposure. In order to effect the alignment, the wafer is provided thereon with a mark for detecting a position (an alignment mark) which has been transcribed thereonto by the exposure in the lithographic process previously carried out, whereby the projection exposure apparatus can detect the position of the wafer or a circuitry pattern on the wafer with high precision by detecting the position of the alignment mark.
Hitherto, an alignment microscope for detecting such an alignment mark may be broken down roughly into an on-axis type for effecting the detection of the alignment mark through a projecting lens and an off-axis type for effecting the detection of the alignment mark without the use of a projecting lens, however, such an alignment microscope of an off-axis type will be more appropriate than that of an on-axis type for a projection exposure apparatus where there is employed an excimer laser light source which will become a mainstream for this purpose from now on. The reasons why the alignment microscope of such an off-axis type can be employed more advantageously than that of an on-axis type are because it allows a wider freedom of optical design without taking any chromatic aberration into account and it can use a variety of alignment systems due to the fact that it is mounted separately from the projecting lens, while the alignment microscope of such an on axis type cannot converge alignment light or cause a greater error due to chromatic aberration even if such alignment light would be converged, because the projecting lens is corrected for the chromatic aberration against exposure light. Further, for example, a phase-contrast microscope or a differential interference microscope may also be employed.
FIG. 10 is an abbreviated plan view showing a portion nearby a wafer table of a conventional projection exposure apparatus with an alignment microscope of an off-axis type. As shown in FIG. 10, the wafer table as a substrate stage or which a wafer W is placed is provided thereon with a Y-axially moving mirror 80Y having a surface reflecting at an angle normal to the Y-axis and an X-axially moving mirror 80X having a surface reflecting at an angle normal to the X axis. The Y-axially moving mirror 80Y is provided with a Y-axial interferometer, although not shown, and Y-axial interferometer beams in the measuring longitudinal axis in the Y-axial direction passing through a projection center of an optical projection system PL and a detection center of an alignment microscope 82 allows the measurement of the Y-axial displacement from the reference position of the Y-axially moving mirror 80Y, thereby determining the Y coordinate of the wafer table. On the other hand, an interferometer for measuring the X-coordinate of the wafer table is provided with an exposing X-axial interferometer for projecting interferometer beams in a measuring longitudinal axis Xe in the X-axial direction passing through the projection center of the optical projection system and with an aligning X-axial interferometer for projecting interferometer beams in a measuring longitudinal axis Xa in the X axial direction passing through the detection center of the alignment microscope 82.
As at least three interferometers are provided for measuring the positions of the wafer table as described hereinabove, the wafer table can be aligned by the aligning interferometer (the measuring longitudinal axis Xa, Y) at the time of alignment and the position of exposure can be measured and determined by the exposing interferometer (the measuring longitudinal axis Xe, Y) at the time of exposure, thereby allowing an accurate alignment and exposure so as to cause no Abbe's error due to the rotation of the wafer table.
As an interferometer for managing the position of the wafer table of a projection exposure apparatus, there has generally been employed a Twyman-Green interferometer. The Twyman-Green interferometer has a fixed light path having an unvariable arm length (a length of the light path), which is a light path of interferometer beams to an unshown fixed mirror, and a moving light path which allows its arm length to vary in accordance with the position of a moving mirror. It is further arranged so as to determine the position of the wafer table as a relative displacement between the fixes mirror and the moving mirror by comparing the arm lengths or the fixed light path and the moving light path. However, this interferometer determines the position of the wafer table by sequentially adding signals of the positions of the moving mirror one by one so that the position of the moving mirror cannot be measured if the interferometer beams were cut and they would not strike the moving mirror. Accordingly, it requires the interferometer beams to always strike the moving mirror. At this end, the conventional projection exposure apparatus is designed in such a manner that in order to effect alignment measurement and exposure over the entire surface of the wafer W, the length Lm of the moving mirror located in the longitudinal length of the wafer table (the X-axially moving mirror 80X as in FIG. 10) should be set so as to establish the relationship as follows: EQU Lm&gt;Dw+2BL
where Dw is the diameter of the wafer; and BL is the distance between the measuring longitudinal axes Xe and Xa. In other words, it is requisite for the length of the X-axially moving mirror to satisfy the above relationship.
Moreover, a wafer size becomes larger as the time passes and the technical innovation develops, and a wafer may recently be as large as 300 millimeters in diameter. Therefore, the length of the moving mirror should also be made longer, resulting in enlarging the size of a wafer table on which the moving mirror is mounted.
Further, since an alignment system for the projection exposure apparatus of an off-axis type has an alignment micro-scope mounted outside the optical projection system, the diameter of the optical projection system should become larger, too, as the N.A. of the optical projection system becomes higher and the field thereof becomes greater. Moreover, as the distance between the optical projection system and the alignment micro-scope becomes apart to a more extent, this causes the moving mirror to becomes longer in length and the wafer table to become greater in size.
The fact that the wafer table as a substrate stage becomes greater in size due to the various factors as described herein-above is now becoming a big issue to solve. In other words, the larger size and the greater weight of the substrate stage may cause the worsening of control and a decrease in throughput, thereby resulting in making the entire size of the apparatus larger and the entire weight thereof heavier.
Under such circumstances, great demands have been made to develop technology of making a substrate stage compact in size and consequently lighter in weight, thereby enabling control over a movement of the substrate stage at a higher speed and effecting alignment at a more accurate way.